Method of making an electron tube



Sept. 1, 1964 F. R. RAGLAND, JR

METHOD OF MAKING AN ELECTRON TUBE 2 Sheets-Sheet 1 Filed Oct. 16, 1962 INVENTOR. FkAA K K final/v4.4: BY

p 1, 1964 F. R. RAGLAND, JR

METHOD OF MAKING AN ELECTRON TUBE 2 Sheets-Sheet 2 Filed Oct. 16, 1962 IN VEN TOR RANK A. F/IGMA/ fi BY Arron/er United States Patent F 3,146,515 METHGD 8F llfAlilNG AN ELECTREBN TUBE Frank R. liagland, .l'n, Lancaster, llan, assignor to Radio Corporation of America, a corporation of Delaware Filed Oct. 16, 1962, Ser. No. 2%,832 Ill Claims. (Cl. 29-25.ll7)

The present invention relates to an improved method of making an electron tube having closely spaced cylindrical electrodes, and particularly to a method of disposing the electrodes in accurate concentric relation to provide a uniform spacing between all portions of active surfaces thereof.

One type of electron tube utilizes two concentric cylindrical electrodes in the shape of perforated cylindrical cups, each cup being supported by a flanged end sealed to a ceramic insulator forming part of the tube envelope. In applications of tubes of this type at ultra-high frequencies it is desirable that the spacing between the two electrodes be relatively small and uniform throughout all portions of the electrodes that are active during tube operation.

Means heretofore employed to secure the desired uniform close spacing between the electrodes have not been fully satisfactory. Difficulty has been encountered as a consequence of an otherwise advantageous method of forming radially registering openings in the two concentric grids. This method as described in U.S. Patent No. 2,980,984, issued April 25, 1961, to Shrader et al., employs two cup shaped electrode blanks mutually fixed in concentric relation prior to the formation of the openings therein. The openings are formed by advancing an erosion zone from one end of the electrode blanks to a region adjacent to the flanged end thereof. For ruggedness, the electrode blank includes an inwardly extending flange at the aforementioned end thereof. The presence of this flange has made it impossible prior to the formation of the openings, to visually examine the interior of the outer blank of the two blanks to determine whether a desired concentricity of the two blanks has been achieved. Attempts to secure the desired concentricity of the close spaced electrode blanks, has therefore involved blind jigging, either by engaging externally available portions of the electrode structures by jigging means, or by interposing jigging means between the electrodes without certainty that the jiggin means is accomplishing its purpose.

In one form of interposed jigging means a plurality of wires are temporarily fixed to the outer surface of the inner electrode blank, and the outer electrode blank is threaded thereover. This is not fully satisfactory in manufacture due to the time required in effecting the temporary fixing of the wires to the inner electrode blank. Furthermore, there is no assurance that the wires will remain fixed after the outer electrode blank has been threaded over the inner blank and into frictional engagement with the wires temporarily fixed to the inner blank.

The inability to determine positively, as by visual inspection, that the two blanks are in desired concentric relation prior to a mutual fixing of the blanks, is costly in manufacture. Failure of required concentricity of the blanks is sometimes determined only after a tube has been completed and tested, in the form of improper tube rating, variation in characteristics from tube to tube, and/ or short tube life.

Another objection to prior spacing techniques to obtain concentricity of two tubular imperforate blanks, has resided in the hazard that a portion of the jigging means may not respond fully to an attempted removal thereof, thus leaving a portion of such means in the completed tube with consequent impairment of tube operation.

Accordingly, it is an object of the invention to provide an improved method of making an electron tube having two closely spaced tubular grids in accurate concentric relation.

It is a further object to provide an improved method of visually and accurately disposing two cylindrical electrode blanks in concentric relation so that the intended active surfaces thereof are uniformly and closely spaced from each other.

It is another object to provide a method of electron tube manufacture, wherein the insertion and removal of jigging means for establishing a desired concentricity of two tubular electrode blanks, are facilitated.

These and other objects are accomplished in accordance with the present invention by providing initial slots in the outer of the electrode blanks, only in regions remote from the intended active surfaces thereof, and prior to fixing the outer blank with respect to the inner blank. The slots so formed constitute access openings to the space between the two blanks when loosely assembled prior to fixing, through which straight wire shims may be readily inserted and removed and the degree of concentricity of the two blanks may be visually checked.

The slots referred to may be formed by the method of electrical discharge machining described in the aforementioned patent and the same electrical broach may be used to provide the initial slots aforementioned, and the completed slots required for grid functions after a mutual fixing of the two electrode blanks. In forming the subsequent and completed slots, teeth of the electrical broach may be caused to register with and enter the initially made slots so as to constitute the subsequent slots continuations of the initial slots. While the initially formed slots are at non active regions of the outer electrode blank, it is preferable to avoid formation of a second group of slots in these regions in the interests of electrode ruggedness. However, it is not essential that the register of the broach teeth with the initially formed slots, in a final slot-making operation, be characterized by a high degree of accuracy, since the initially formed slots are at a region that is not critical to tube operation. The fact that an accurate register of the broach teeth with the initial slots, is not required, is of appreciable advantage in manufacture.

Further objects and features will become evident as the present description continues.

In the drawing, to which reference is now made for an example of the invention:

FlG. l is a sectional elevation of an imperforate cupshaped electrode blank in position with respect to an electrical broach, for the formation therein of a group of initial slots;

FIG. 2 is a perspective view of the upper portion of the electrode blank shown in FIG. 1 and depicts the charactor of the initial slots formed in the blank;

FIG. 3 is a sectional elevation of parts, including the electrode blank shown in FIG. 2, mounted on a brazing jig for oven heating and a mutual fixing of the parts;

FIG. 4 is a top view of the electrode blank illustrated in FIG. 3, and shows the ports or openings provided by the initially formed slots therein, and which expose the outline of an inner cup-shaped electrode blank for determining the concentricity of the two blanks, and permit the insertion of spacing means between the two blanks; and

FIG. 5 is an elevation partly cut away, showing the completed electrodes in operative position in. an electron tube.

An electron tube of the type shown in FIG. 5, comprises a tubular cathode 10, a tubular control grid 12, a tubular screen grid 14, and a tubular anode 16. In tubes of this type designed for ultra high frequency operation, several factors are deemed of critical significance.

For good operation of a tube of this type the openings in the control grid 12 should be very accurately aligned radially with corresponding openings in the screen grid 14. Such accurate alignment can be secured by subjecting the two grids in imperforated blank form to a method of electrical dischcrge machining described in the aforementioned US. patent. Prior to forming the openings in the grid blanks, the grid blanks should be disposed in accurate concentric relation. This is a problem that has not been solved fully heretofore.

One of the difliculties in finding a satisfactory solution to this problem is caused by the structure of the grid blanks. The structure of a grid blank 18 as shown in FIG. 1, includes imperforated cylindrical walls 29 and an imperforate inwardly extending flange 22 to one end of the structure. The imperforated flange 22 is used in order to render the completed grid of suflicient ruggedness for service in an electron tube. As will be noted by reference to the aforementioned U.S. patent, an electrical discharge machining operation for providing radially registering openings in the concentric grid blanks, is most conveniently effected by starting the erosion at the top of a grid blank, the electrical broach employed extending inwardly to perforate a portion only, of the inwardly extending flange 22. This leaves an imperforate portion of the flange for service as a ruggedizing means, that is, it ties in adjacent ends of the Wires or strips formed by the broach. Without such tie-in, the adjacent ends referred to'would be free of any support and no restraint would be available to erratic movements of such free ends with respect to each other and other tube parts. Such erratic movements would be detrimental to tube operation and might render a tube inoperative.

While it is therefore desirable to include the inwardly extending flange 22, in the structure of the blank 18, shown in FIG. 1, the presence of this flange has caused difiiculty in securing a desired concentricity of grid blank 18 with respect to an inner grid blank 24 shown in FIG. 3. Not only does the imperforate flange 22 preclude a convenient introduction of a spacing means, but it also blocks a visual inspection of the spacing between the two grid blanks. Such visual inspection would permit a visual check as to the concentricity of the two blanks, and is highly desirable in the interests of economical manufacture of a tube. In the absence of such inspection, a failure of the spacing means may go undetected until the tube is completed and tested, and may necessitate a scrapping of the entire tube.

This difficulty is avoided by a practice of the present invention. In accordance with one way of practicing the instant invention, an imperforate grid blank is provided with an initial group of slots by practicing a step illustrated in FIG. 1. In carrying out this step, an imperforate grid blank 18 having cylindrical sides 20 and an inwardly extending flange 22, is supported in a jig 26. An electrical broach 28 having a plurality of inwardly extending teeth 30 is caused to move downwardly towards the blank 18. As the broach 28 gets close to the blank 18, a series of electrical discharges between the teeth of the breach and the blank 18, cause erosion in the blank to form slots 32 therein as shown in FIG. 2.

The slots extend inwardly of the flange 22, a distance at least as great and preferably greater than the distance to be observed between the cylindrical walls 2th of the grid blank 18 and similar walls 33 of the inner grid blank 24, as shown in FIG. 3. In this way, the slots 32 serve not only as openings through which spacing means may be inserted between the two grid blanks, as will be explained, but also permit a visual inspection of the space between the two blanks to determine the concentricity thereof.

The downward extent of the slots 20 along the walls 20 of the grid blank, should not be of such magnitude as to dispose the slots in the eventual action region of the grid. The reason for this will become apparent as the description proceeds. It is feasible in practicing the invention to make the slots along the walls 20, only as long as the thickness dimension of the flange 22, to assure complete perforation of the flange. As a manufacturing convenience, however, the slots 32 are made slightly longer, but not long enough to extend into the active portion of the eventual grid.

Another desirable characteristic of the slots 34, is that each slot should have a width at least equal to the space separating the two grid blanks 24, 34 shown in FIG. 3. This will permit insertion into a slot of a wire of circular cross-section and having a diameter for snugly fitting into the space between the blanks 24, 34.

After the grid blank 18 has been provided with the initial slot 32, to form a partly processed grid blank 34, the blank 34 is mounted with other tube parts on a brazing jig as as shown in FIG. 3. The other tube parts comprise the inner grid blank 24 over which the blank 34 is teleseoped; a cathode flange 38; and a plurality of ceramic rings including ring 40 between cathode flange 38 and flange 42 of the grid blank 33, a ring 44 between the flange 42 and a flange of the grid blank 34, and a ring 43 resting on flange 46 for later sealing to the anode 16 of the tube shown in FIG. 5.

While the parts referred to are loosely assembled on jig 36, FIG. 3, a plurality of gauging members, which may be in the form of wires 50 of circular cross-section, are inserted into a group of slots 32, that are angularly equally spaced, and extended into the space between electrode blanks 24, 34 shown in FIG. 3. If the blanks 24, 34 should initially be out of concentric relation the loose mounting of the blanks on jug 36, will leave blank 34 free to move to a position to which it is urged by the gauging means. As shown in FIG. 4, in one example, four gauging members 50 are employed and are spaced abut Since as indicated in the foregoing the gauging members have a diameter for substantially snugly fitting into the space between the blanks 24, 34 when the blanks are in accurate concentric relation, the insertion of all four members 50 into the aforementioned space will be accompanied by a force sufllciently strong to move the loosely mounted blanks into concentric position.

In one example, each gauging wire 50 had a diameter of 10 mils, and the spacing between the two grid blanks 24, 34 was 11 mils.

While four gauging members are shown by way of example, a larger number may be used. As few as three is tolerable for the function of bringing the two blanks 24, 34 into concentric relation.

To avoid a brazing of the jig 36 and the gauging members St? to work pieces being processed, the jig 36 and members 5ft should be made of materials that do not respond to brazing, such as oxidized stainless steel.

It will be noted in FIG. 3 that the lower flanged end portion of the electrode blank 34, is free to move transversely without restraint by the jig 36, the lower end portion of blank 24 being restrained by a cylindrical portion 53 of jig 35. Since such end portions should also be concentric, the gauging members 50 should be extended downwardly into the space between blanks 24, 34 a sufficient distance to also render the end portions of the blanks concentric. In one example found satisfactory for this purpose, the gauging members 5b were extended downwardly to a region 52, as shown in FIG. 3 which is approximately at the termini of the cylindrical portions of the blanks. It is desirable that the gauging members Stl be sufliciently long so that after extension thereof to the region 52, manually engageable portions will remain above the top of blanks 24, 34 to permit manual removal of the members by simply pulling the same from the blanks 24, 34 after the tube parts shown in FIG. 3 have been fixed as by brazing.

To facilitate brazing, the surfaces of ceramic rings 40, 44, and 43 engaging the metal flanges 38, 42 and 46 are suitably metalized.

After the tube parts shown in FIG. 3 have been brazed by exposure to the ambient of a suitable oven, the two electrode blanks 24, 34 of the fixed assembly, are subjected to a further electrical discharge machining operation. In performing this further operation, the same electrical broach 28 that was used to form the initial slots 32, is employed.

The first step in this further eletcrical discharge machining operation involves disposing the broach 28 above and in coaxial relation with respect to the fixed electrode blanks 24, 34 with the teeth 30 of the broach in substantial alignment with the initial slots 32 in the blank 34-. The broach 28 is then lowered to engage the initial slots. The alignment need be only of suflicient accuracy to avoid removal of any substantial portion of the flange 22 between the slots 32, as shown in FIGS. 2 and 4. A satisfactory alignment can be obtained by actually inserting the teeth of the broach into the initial slots.

If some small portion of the flange 22 between the slots 32 should be removed by the further machining step, it is not harmful to tube operation. This is for the reason that the initial slots 32 extend downwardly such a short distance that they are remote from the active region of the eventual grids. Thus, as shown in FIG. 5, the upper end of the active regions of the two grids 12, 14 is spaced downwardly of the grids to a location at or slightly below the upper end of the cathode lltl. If the cathode 10 were added to the structure shown in FIG. 3, the eventual active region of the blanks 24, 34 would be substantially the region encompassed by the brace 54. Therefore any slight deformation of the blank 34 at the region of the initial slots 32, resulting from a slight misregister between the broach teeth 36 and the slots 32, is not harmful to tube operation.

The second step involves downward travel of the broach 28 in an electrical discharge machining operation until the active portion 54 of the two grid blanks 24, 34 have been eroded to provide a plurality of axially parallel and radially registering slots or openings 56, 57 (FIG. therein. Except for the requirement that the broach teeth 3% observe a substantial register with the initial slots 32, the further operation may otherwise be performed in the manner described in the aforementioned US.

patent. I

After the radially registering openings or slots 56, 57 (FIG. 5) have been formed in the active portions of the two grid blanks 24, 34 the assembly comprising the parts shown in FIG. 3 after fixing and electrical discharge machining, is incorporated in the electron tube structure shown in FIG. 5. Such incorporation involves sealing the anode 16 to the ceramic ring 48, and the extension of a cathode and heater sub-assembly (not shown) through the opening provided by flange 38 and into the space defined by the fully processed inner grid blank 24 and in position to be in radial register with the active portion of the blank. The cathode-heater sub-assembly includes a flange (not shown) that snugly engages the inner Wall of flange 38, and to which it is suitable sealingly fixed as by welding. The resultant electron tube envelope formed by assembling and joining the sub-assemblies thereof is aforementioned, is evacuated through exhaust tubulation 56.

What is claimed is:

1. Method of making an electron tube comprising the steps of:

(a) simultaneously forming a plurality of preliminary slots in a side portion adjacent to one end of a tubular cup-shaped grid blank, said side portion being remote from an eventual active region of the completed grid, and in an inturned flange at said one end at least a predetermined distance inwardly of the periphery of said flange and less than a radial dimension of said inturned flange,

(b) loosely mounting said slotted grid blank in a subassembly comprising said blank and a second tubular grid blank within said slotted grid blank, said second blank having transverse dimensions of such magnitude that when said second blank is in concentric relation with respect to said slotted blank, said blanks are mutually spaced said predetermined distance,

(0) inserting into some only, of said slots and into the space between said grids in radially parallel relation with respect to said blanks, a plurality of gauging members each having a transverse magnitude substantially equal to said distance, whereby said loosely mounted blank is urged into concentric relation with respect to said second blank, and the concentricity may be checked by visual observation through the other of said slots,

(d) mutually fixing said grid blanks in said concentric relation,

(0) simultaneously forming axially parallel slots in said two grid blanks in the eventually active regions of said blanks and substantially as continuations of said preliminary slots,

(f) sealing a tubular anode to said subassembly with the eventual active regions thereof coextensive with the eventual active regions of the slotted grid blanks, and

(g) extending a cathode heater assembly into said second slotted grid blank in substantial radial register with said eventual active regions.

2. Method of making an electron tube comprising the steps of:

(a) forming at least three preliminary slots substantially equally spaced angularly, in one end portion of a tubular cup shaped grid blank, wherein said slots terminate in a region remote from an eventual active region or" the completed grid, said slots including a portion extending across an inturned flange at said one end portion at least a predetermined distance and less than a radial dimension of said inturned flange,

(b) loosely mounting said slotted grid blank in a subassembly comprising said blank and a second tubular grid blank Within said slotted grid blank, wherein said blanks, when in concentric relation are mutually spaced said predetermined distance,

(c) inserting at least three gauging members into said slot portions and into the space between said grids in axially parallel relation with respect to said blanks and equidistantly angularly spaced, whereby said loosely mounted blank is urged into concentric relation with respect to said second blank,

(d) mutually fixing said grid blanks insaid concentric relation,

(e) simultaneously forming axially parallel slots in said two grid blanks in the eventually active regions of said blanks and substantially as continuations of said preliminary slots,

(f) sealing a tubular anode to said sub-assembly with the eventual active region thereof coextensive with the eventual active regions of the slotted grid blanks, and

(g) extending a cathode heater assembly into said second slotted grid blank in substantial radial register with said eventual active regions.

3. Method of making an electron tube comprising the steps of:

(a) forming a plurality of preliminary slots radially across an inturned flange at one end of a tubular cup-shaped grid blank, said slots extending at least a predetermined distance across said flanges and less than a radial dimension of said inturned flange,

(b) loosely mounting said slotted grid blank in a subassembly comprising said blank and a second tubular grid blank within said slotted grid blank, wherein said blanks when in concentric relation are mutually spaced said predetermined distance,

(c) inserting a plurality of gauging members into said slots and into the space between said grids in radially parallel relation with respect to said blanks, said gauging members having transversed dimensions for causing the loosely mounted blank to be urged into concentric relation with respect to said second blank,

(d) mutually fixing said grid blanks in said concentric relation,

(2) simultaneously forming axially parallel slots in :said two grid blanks in the eventually active regions of said blanks and substantially as continuations of said preliminary slots,

(1) sealing a tubular anode to said sub-assembly with the eventual active regions thereof coextensive with the eventual active regions of the slotted grid blanks, and

(g) extending a cathode heater assembly into said second slotted grid blank in substantial radial rejector with said eventual active regions.

4. Method of making an electric tube having a plurality of concentric tubular electrodes including a cathode, an anode and two grids between said cathode and anode, comprising the steps of:

(a) cutting an initial axially parallel slot in an end portion of an imperforate tubular grid blank, spaced from an eventual active portion of the completed grid,

"(11) loosely mounting said initially slotted grid blank with another imperforate tubular grid blank telescoped therein and in spaced relation therewith,

(c) inserting a gauging wire into said initial slot and into the space between said grid blanks, said wire having a diameter substantially equal to the spacing between said blanks when said blanks are concentric, whereby said blanks are disposed in concentric relation,

(d) mutually fixing said blanks while said wire is in said space,

(a) forming simultaneously a continuation of said initial slot in the eventual active region of said initially slotted blank and in radial register,

( fixing said anode in concentric relation with respect to the active region of said initially slotted grid, and

"'(g) fixing said cathode in concentric relation with respect to the active region of said another grid.

5. In a method of making an electron tube of the tetrode variety, the steps of:

(a) forming initial slots in an end portion of a first cup-shaped grid blank having a cylindrical portion and an inturned flange at one end thereof, said slots extending continuously at least a predetermined distance across a portion only of said flange and a relatively short distance along said cylindrical portion and spaced from the eventual active region of said blank, said slots having a width at least as great as the length of said predetermined distance,

(b) loosely mounting said first cup-shaped grid blank in approximate concentric relation with a said second cup-shaped grid blank disposed therewithin, said second blank having a diameter for spacing said second blank from said first blank said predetermined distance when said blanks are in concentric relation,

() gauging the spacing between said blanks through said slots and moving said blanks into substantially concentric relation,

(d) mutually fixing said blanks in said substantially concentric relation, and

(e) forming radially registering slots in the eventual active regions of said blanks in radial and axial register with said initial slots.

6. In a method of making an electron tube of the tetrode variety, the steps of:

(a) forming at least three initial slots in an end portion of a first cup-shaped grid blank having a cylindrical portion and an inturned flange at one end thereof, said slots extending continuously at least a predetermined distance across a portion only of said flange and spaced from the eventual active region of said blank, said slots having a width at least as great as the length of said predetermined distance,

(b) loosely mounting said first cup-shaped grid blank in approximate concentric relation with a second cup-shaped grid blank disposed therewithin, said second blank having a diameter for spacing said second blank from said first blank said predetermined distance when said blanks are in concentric relation,

(0) extending at least three gauging members between said blanks through three equidistantly spaced ones of said slots and moving said blanks into substantial concentric relation,

(d) mutually fixing said blanks in said substantial concentric relation, and

(e) forming radially registering slots in the eventual active region of said blanks in axial register with said initial slots.

7. Method of making and mounting a tubular grid having a partly closed end, comprising:

(a) forming in a grid blank initial slots extending radially across a portion of the closure means at said partly closed end,

(b) inserting gauging members through said slots for moving and spacing said grid blank a predetermined distance from another electrode,

(0) mutually fixing said grid blank and said another electrode while engaged by said gauging members, and

(d) forming continuations of said initial slots in the sides of said grid blank to complete said grid.

8. Method of making and mounting a tubular grid having a partly closed end, comprising:

(a) forming in a grid blank initial slots extending radially across a portion of the closure means at said partly closed end,

(b) inserting cylindrical gauging members through said slots for moving and spacing said grid blank a predetermined distance from another electrode, said slots having a width at least equal to said predetermined distance,

(c) mutually fixing said grid blank and said another electrode while engaged by said gauging members, and

(d) forming continuations of said initial slots in the sides of said grid blank to complete said grid.

9. Method of making an electron tube having a plurality of concentric tubular electrodes including a cathode, an anode and two grids between said cathode and anode and wherein the outer of said grids includes an inturned flange at one end thereof, comprising the steps of:

(a) cutting an initial slot through a portion of said inturned flange in register with an annular space between said grids in the completed tube, said slot having a width at least equal to the radial dimension of said space,

(b) loosely mounting said initially slotted grid blank with another imperforate tubular grid blank telescoped therein and in spaced relation therewith,

(c) inserting a gauging wire into said initialslot and into the space between said grid blanks for disposing said blanks in concentric relation, said wire having a diameter substantially equal t the Spacing tween said blanks when said blanks are concentric,

( ly fixing said blanks while said wire is in said spac g,

( fing said wire from said space,

( ning simultaneously a continuation of said initial 'siot in the eventual active region of said initially atted blank and in the active region of said another 'oiank, to provide inner and outer grids,

1g) fixing said anode about and in concentric relation with respect to the active region of said outer grid, a d

(h) fixing said cathode within and in concentric relation with respect to the active region of said inner grid.

10. Method of making a sub-assembly for an electron tube wherein the sub-assembly comprises two concentric tubular grids having radially registering openings therein, comprising:

(a) forming at least three preliminary openings in an end portion of a first tubular grid blank including an inturned flange partly closing the end of said blank at said end portion,

(b) loosely mounting in approximate concentric relation said first blank and a second tubular grid blank having a transverse dimension smaller than that of the first grid blank so as to provide a predetermined spacing therebetween when said blanks are in concentric relation,

(c) extending three gauging members having trans- 1&0 verse dimensions substantially equal to the magnitude of said predetermined spacing through three equidistantly spaced of said preliminary openings and into the space between said blanks, whereby said members urge and move said blanks into concentric relation, (d) mutually fixing said blanks in said concentric relation, and thereafter (e) simultaneously forming radially registering openings as continuation of the said preliminary openings in both of said blanks.

References Cited in the file of this patent UNITED STATES PATENTS 2,446,692 Collins Aug. 10, 1948 2,482,990 Olson Sept. 27, 1949 2,758,361 Kohl Aug. 14, 1956 2,917,811 Powers Dec. 22, 1959 

1. METHOD OF MAKING AN ELECTRON TUBE COMPRISING THE STEPS OF: (A) SIMULTANEOUSLY FORMING A PLURALITY OF PRELIMINARY SLOTS IN A SIDE PORTION ADJACENT TO ONE END OF A TUBULAR CUP-SHAPED GRID BLANK, SAID SIDE PORTION BEING REMOTE FROM AN EVENTUAL ACTIVE REGION OF THE COMPLETED GRID, AND IN AN INTURNED FLANGE AT SAID ONE END AT LEAST A PREDETERMINED DISTANCE INWARDLY OF THE PERIPHERY OF SAID FLANGE AND LESS THAN A RADIAL DIMENSION OF SAID INTURNED FLANGE, (B) LOOSELY MOUNTING SAID SLOTTED GRID BLANK IN A SUBASSEMBLY COMPRISING SAID BLANK AND A SECOND TUBULAR GRID BLANK WITHIN SAID SLOTTED GRID BLANK, SAID SECOND BLANK HAVING TRANSVERSE DIMENSIONS OF SUCH MAGNITUDE THAT WHEN SAID SECOND BLANK IS IN CONCENTRIC RELATION WITH RESPECT TO SAID SLOTTED BLANK, SAID BLANKS ARE MUTUALLY SPACED SAID PREDETERMINED DISTANCE, (C) INSERTING INTO SOME ONLY, OF SAID SLOTS AND INTO THE SPACE BETWEEN SAID GRIDS IN RADIALLY PARALLEL RELATION WITH RESPECT TO SAID BLANKS, A PLURALITY OF GAUGING MEMBERS EACH HAVING A TRANSVERSE MAGNITUDE SUBSTANTIALLY EQUAL TO SAID DISTANCE, WHEREBY SAID LOOSELY MOUNTED BLANK IS URGED INTO CONCENTRIC RELATION WITH RESPECT TO SAID SECOND BLANK, AND THE CONCENTRICITY MAY BE CHECKED BY VISUAL OBSERVATION THROUGH THE OTHER OF SAID SLOTS, (D) MUTUALLY FIXING SAID GRID BLANKS IN SAID CONCENTRIC RELATION, (E) SIMULTANEOUSLY FORMING AXIALLY PARALLEL SLOTS IN SAID TWO GRID BLANKS IN THE EVENTUALLY ACTIVE REGIONS OF SAID BLANKS AND SUBSTANTIALLY AS CONTINUATIONS OF SAID PRELIMINARY SLOTS, (F) SEALING A TUBULAR ANODE TO SAID SUBASSEMBLY WITH THE EVENTUAL ACTIVE REGIONS THEREOF COEXTENSIVE WITH THE EVENTUAL ACTIVE REGIONS OF THE SLOTTED GRID BLANKS, AND (G) EXTENDING A CATHODE HEATER ASSEMBLY INTO SAID SECOND SLOTTED GRID BLANK IN SUBSTANTIAL RADIAL REGISTER WITH SAID EVENTUAL ACTIVE REGIONS. 